1. Field of the Invention
The present invention relates to a light beam scanner which uses a semi-conductor laser as a light source and scans with a laser beam by means of a mechanical deflector such as a rotating polygonal mirror or the like.
2. Description of the Prior Art
In a light beam scanner using a rotating polygonal mirror as a light deflector, it is not possible to prevent the reflecting surfaces of the rotating polygonal mirror from being tilted relative to its rotating axis due to an error in manufacture, resulting in non-uniformity of scanning line pitch. If an attempt is made to remove such an error by improving the precision of the rotating polygonal mirror, an extremely high cost results.
For this reason, there have been introduced optical systems which have a so-called falling compensation function in which a reflecting surface and a scanning surface are placed in a geometrical-optically conjugative relation with respect to a focussing lens system, in a so-called sub-scanning direction, which is parallel with the rotating axis of the rotating mirror. In these optical systems, optical elements which are different in refractive power relative to the scanning surface and the surface perpendicular thereto, that is, a cylindrical lens or a toroidal lens, are introduced. The deflection surface and the surface to be scanned are placed in a conjugative relation with respect to a composite system of these lens systems and the focussing lens in a direction at a right angle to the scanning direction, that is, in a so-called sub-scanning direction. In this case, a condensing lens is disposed so that a light beam from the light source is condensed on the deflecting surface. On the other hand, the light from the light source is incident as a parallel light flux upon the deflecting surface, and the scanning surface is scanned by the rotation of the deflecting surface. For this reason, a cylindrical lens is also introduced into the condensing lens system, and generally a linear spot in a scanning direction is formed on the deflecting surface.
In the past, in the optical system as described above, the linear spot is necessarily formed on the deflecting surface as described above and, therefore, a fixed spacing is required between the light source and the deflector, which limits miniaturization of the apparatus.
On the other hand, there has been proposed an arrangement (Japanese Patent Application Laid-Open No. 59152/79) wherein a linear spot is not made on a reflecting surface but a cylindrical lens having a refractive power in a sub-scanning direction is arranged in front of a deflector to project emitting light fluxes on the deflecting surface in the sub-scanning direction. The light fluxes are made into substantially parallel light fluxes by a focussing lens, which are then focussed on the scanning surface by a convex cylindrical lens having a short focal distance.
In this proposal, the falling compensation effect merely utilizes the shortness of the focal distance of the convex cylindrical lens and therefore, the compensation effect is not perfect. In addition, the light source is limited to a laser which emits parallel light flux and cannot be used for a laser light source of spot luminescense such as a semi-conductor laser.
Further, in a Gauss beam such as a laser beam, when a parallel beam is incident on the lens, the relation between a radius .omega..sub.o of a beam waist at a focussing position and a radius .omega..sub.a of a beam incident on the lens is expressed by ##EQU1## where .lambda. is the wave length, and f is the focal length of the lens. Thus, it is necessary to adjust the radius of incident beam to .omega..sub.a in order to obtain a spot size .omega..sub.o as required. Also, referring to FIG. 4, the following relations are obtained: ##EQU2## where f is the focal length of the lens, .omega..sub.1 is the radius of beam waist on the object side, D.sub.1 is the position relative to the lens, .omega..sub.2 is the radius of beam waist on the image side, and D.sub.2 is the position thereof. Thus, the diameter of spot obtained on the scanning surface by the light source and the optical system is determined constant but if there is irregularity in emitting size, such as from a semi-conductor laser, the spot size of the beam on the scanning surface also results in irregularity. For this reason, it has been proposed to insert an afocal zoom lens system or the like into an optical system to control the spot size (for example, Japanese Patent Application Disclosure No. 56779/79). However, incorporation of two sets of cylindrical afocal zoom lenses in order to control the beam diameter in the main and sub scanning directions as described hereinbefore requires a number of cylindrical lenses, resulting in a higher cost and in a large-size scanning apparatus.